[RFC] Gathering/observing data in the background and foreground

[JulianKingman]

This feature will add background observers for an arbitrary number of types of data. Below are my notes and ideas for how to implement this. Your comments and ideas are welcome.

I propose the following:

1. The app can be initialized with an additional option: observe E.g:

   let options = {
   permissions: {
   read: ["Height", "Weight", "StepCount", "DateOfBirth", "BodyMassIndex"],
   write: ["Weight", "StepCount", "BodyMassIndex"],
   },
   observers: [
   { type: "StepCount" },
   { type: "Weight" }
   ]
   };
   AppleHealthKit.initHealthKit(options);

2. Observers (HKObserverQuery) run in the background, saving data to AsyncStorage, perhaps to the RNAppleHealthkit key. To retrieve it later you could run:

   const healthKitData = await AsyncStorage.getItem('RNAppleHealthkit');
   const steps = healthKitData.StepCount;
   // `steps` might contains something like:
   // [
   //    { date: '1 1 2019', value: 10000}
   // ]

3. To make use of this most effectively, use react-native-background-fetch

   
   BackgroundFetch.configure({
   minimumFetchInterval: 15, // <-- minutes (15 is minimum allowed)
   }, () => {
   
   // Retrieve data
   const healthKitData = await AsyncStorage.getItem('RNAppleHealthkit');
   
   // Stop if there isn't any
   if (!healthKitData)  {
   BackgroundFetch.finish(BackgroundFetch.FETCH_RESULT_NO_DATA);
   }
   
   // Get steps
   const steps = healthKitData.StepCount;
   
   // Upload them
   await fetch('http://placetouploadsteps.com/steps', { method: 'POST', body: JSON.stringify({ steps });
   
   // No need to waste storage
   AsyncStorage.removeItem('RNAppleHealthkit');
   
   // Finish
   BackgroundFetch.finish(BackgroundFetch.FETCH_RESULT_NEW_DATA);

}, (error) => { console.log("[js] RNBackgroundFetch failed to start"); });

4. Event triggers still work, if your app is in the foreground (but with a simpler API)
Per https://github.com/terrillo/rn-apple-healthkit/blob/master/docs/initStepCountObserver().md
```javascript
import { EventEmitter } from 'rn-apple-healthkit';
EventEmitter.addListener(
  'change:StepCount',
  (evt) => {
    const steps = evt.data;
  }
);

[EJohnF]

I don't have time to find related page in healthKit docs, but I remember that it isn't possible to use react-native-background-fetch to get healthKit data while app in Background. We have to use a built-in HK mechanism for it.

[JulianKingman]

Yes, that’s true of getting the data. That’s why in step 2 it retrieves the data in the background (using healthkit’s HKOvserverQuery, the built-in mechanism) and stores it to AsyncStorage, because (I’m pretty sure) AsyncStorage is available with background fetch.

[adcuz]

AFAIK background fetch doesn't run if the device is locked.